Hi guys. I am currently studying Sub-atomic physics and it seems to me that there are different formulas that work in different frame of reference. In particular, I would like to ask: 1. Which frame should we use in applying Fermi's Golden Rule? (I've read somewhere that the matrix element is invariant, but I assume the density of states must be frame dependent) 2. The resolution of a probe is defined by its de Broglie wavelength. Once again, this is frame dependent, so which frame should we use to determine a probe's resolution. 3. Is there a general rule of thumb when deciding which frame to use when applying a formula? (It seems to me that most of the problems are done in Zero momentum frame, or is it?) Thank you so much guysss!